An exemplary traveling wave tube (TWT) 20 is illustrated in FIG. 1. The elements of TWT 20 are generally coaxially arranged along a TWT axis 22. They include an electron gun assembly 24, a slow wave structure (SWS) 26, a beam focusing arrangement 28 which surrounds SWS 26, a microwave signal input port 30 and a microwave signal output port 32 which are coupled to opposite ends of SWS 26, and a collector assembly 34. A housing 36 is typically provided to protect the TWT elements.
In operation, electron gun assembly 24 launches a beam of electrons into SWS 26. Beam focusing arrangement 28 guides the beam of electrons. A microwave input signal 38 is inserted at input port 30 and moves along SWS 26 to output port 32. SWS 26 causes the phase velocity (i.e., the axial velocity of the signal's phase front) of the microwave signal to approximate the velocity of the electron beam.
As a result, the beam's electrons are velocity modulated into bunches which overtake and interact with the slower microwave signal. In this process, kinetic energy is transferred from the electrons to the microwave signal; the signal is amplified and is coupled from output port 32 as an amplified microwave output signal 40. After their passage through SWS 26, the beam's electrons are collected in collector assembly 34.
Electron gun assembly 24, SWS 26, and collector assembly 34 are again shown in the TWT schematic of FIG. 2. Electron gun assembly 24 has a cathode 42 and an anode 44. Collector assembly 34 has a first collector stage 46, a second collector stage 48, and a third collector stage 50.
SWS 26 and body 52 of TWT 20 are at ground potential. Cathode 42 is biased negatively by a voltage V.sub.cath from a cathode power supply 54. An anode power supply 56 is referenced to cathode 42 and applies a positive voltage to anode 44. This positive voltage establishes an acceleration region 58 between cathode 42 and anode 44. Electrons are emitted by cathode 42 and accelerated across acceleration region 58 to form electron beam 60.
As described above with reference to FIG. 1, electron beam 60 travels through SWS 26 and exchanges energy with a microwave signal which travels along the SWS from input port 30 to output port 32. Only a portion of the kinetic energy of electron beam 60 is transferred in this energy exchange. Most of the kinetic energy remains in electron beam 60 as it enters collector assembly 34. A significant part of this kinetic energy can be recovered by decelerating the electrons before they are collected by collector assembly 34.
Electron deceleration is achieved by application of negative voltages to collector assembly 34. The potential of collector assembly 34 is "depressed" from that of TWT body 52 (i.e., made negative relative to the TWT body). The kinetic energy recovery is further enhanced by using a multistage collector, e.g., collector assembly 34, in which each successive stage is further depressed from the body potential of V.sub.B. For example, if first collector stage 46 has a potential of V.sub.1, second collector stage 48 has a potential of V.sub.2 and third collector stage 50 has a potential of V.sub.3, these potentials are typically related by the equation V.sub.B =0&gt;V.sub.1 &gt;V.sub.2 &gt;V.sub.3, as indicated in FIG. 2. A collector power supply 62 applies voltages V.sub.1, V.sub.2, and V.sub.3 to depress the respective collector stages.
Collector assembly 34 includes a ceramic isolator or insulator to electrically isolate the collector stages from TWT body 52. The collector stages must be isolated from one another because they have different voltage potentials. Accordingly, the collector stages are secured to the inner surface of the isolator. The isolator also assists in dispersing the heat created by the electrons striking the collector stages. The outer surface of the isolator is secured to a sleeve. The sleeve forms a vacuum envelope for collector assembly 34. The sleeve is typically fabricated of a metal or a metal/ceramic composite assembly.
Cathode 42 and anode 44 are also biased at a different voltages than TWT body 52. Therefore, a ceramic isolator or insulator is used as part of the mechanical structure of electron gun assembly 24 to electrically isolate cathode 42 and anode 44. The isolator also disperses heat created by cathode 42.
Electron gun assembly 24 operates within a vacuum envelope. To form the vacuum envelope, a sleeve is placed around the isolator, the cathode, and the anode. The sleeve is typically fabricated of a metal or a metal/ceramic composite assembly.
In typical TWTs, brazing and welding are used to connect the sleeves, the isolators, the collector stages, the anode, and the cathode of the electron gun and collector assemblies. In particular, the sleeves are brazed or welded to the outer surfaces of the isolators. The collector stages, the anode, and the cathode are brazed or welded to the inner surfaces of the sleeves. Furthermore, electrode lead connections may be brazed or welded to the collector stages, the anode, the cathode, and the isolators.
Copper, gold, or silver alloys are used for brazing or welding. A primary disadvantage with these methods is that they limit the types of materials that may be used for the cathode, anode, collector stages, lead connections, isolators, and sleeves. Furthermore, these components must be prepared extensively so that they have proper brazing and welding surfaces. For instance, it is often necessary to use a multiple brazing process with alloys, such as active braze alloys, having different melting points or material compatibilities.
These methods are disadvantageous because the vacuum pressure within the electron gun and collector assemblies or the electrical resistivity of the isolators may be adversely affected by the components being exposed to multiple processes. These methods are also disadvantageous because the different thermal expansion rates of the alloys may limit the operating temperature range of the TWT.
Thus, electron gun and collector assemblies fabricated without brazing or welding and in a minimum number of steps is needed.